Prior Art
For some time, image intensifier tubes have been used in a variety of applications for direct viewing at low light levels and near infrared regions of the spectrum. Image intensifier tubes have been used in a variety of military, scientific and industrial applications where assistance in viewing objects at low light levels is necessary. For example, the devices are used for telescopic observation of stellar bodies or in military applications to view dimly illuminated targets.
Image intensifier tubes are electro-optical devices which convert a low energy visible or invisible radiant image into an electron image by means of a photocathode. This image is increased in energy and reconstructed by a focusing electric field on a phosphor screen or a microchannel plate electron multiplier positioned in front of a phosphor screen. The radiant image is reconverted on the phosphor screen to a brighter image of like or varied size.
The development of PIP image intensifier assemblies has progressed through three generations of units. The so called zero generation tubes were primarily infra-red tubes, internally processed. The performance and quality of these tubes were limited, and compact high gain devices could not be accomplished.
In first generation image intensifier tubes, the low light level image is incident upon a fiberoptic face plate which focuses the image on a photocathode where the photon image is converted into an electronic one. The electrons are accelerated toward a phosphor screen, while the spatial information is maintained by the electron optics. The accelerated electrons strike the phosphor, thus inducing an amplified image. Generally, three stages of intensifier stages are utilized in the first generation type.
Although the first generation of image intensifier tubes were impressive, certain deficiencies resulted from cascading the three stages of amplification. These deficiencies included: the added features of distortion and vignetting at each stage; the effective lengthening of the phosphor decay time, which produces streaking when a scene containing bright lights is viewed; and bright sources within the field of view causing blooming in each stage which can "wash out" the entire scene in severe cases.
After many years of development, a second generation image intensifier tube was developed. This second generation unit incorporated a microchannel plate comprised of a bundle of discrete hollow glass tubes or channels capable of amplifying an electron image by many orders of magnitude. As in the first generation of image intensifier tubes, the electron image in the second generation units is generated by a photocathode in response to the incident radiation image. However, the multiplied electron image from the microchannel plate is directed onto a phosphorus screen for providing an intensified display of the sensed radiation image without the need for stages of amplification.
Because the second generation tube produced sufficient gain in a single stage, streaking, distortion and vignetting was substantially reduced. Further, the ability of the microchannel plate to localize high current regions, resulting from bright sources, provides a system which reduces blooming and "wash out", resulting in a better contrast rendition through the system. Further, a single stage construction requires substantially less intensifier power than is required by the first generation image intensifier tubes.
In both the first and second generation image intensifier tubes of the internally processed type, the photocathode is formed by admission of an appropriate antimony metal and metal alkali vapor into the evacuated housing to the photocathode region through side arms mounted on the housing. A similar side arm connects to a getter wire to accomplish the deposition of the material for gas absorption. These side arm appendages are "pinched off" and removed after formation of the photocathode. The extension of the pinched off side arms generally dictates the minimum diameter for packaging the tube.
The zero generation image intensifier assemblies define a circumference slightly smaller than the circumference of both the first and the second generation image intensifier tubes. As a result, the prior art second generation intensifier tubes cannot be directly substituted into devices originally designed to receive the zero generation tubes. Thus, use of the improved first and second generation image intensifier tubes on devices originally designed to accept the zero generation tubes has required a complete reworking of the devices to accept the larger diameter first and second generation tubes.
Other image intensifier assemblies, such as the image intensifier commonly known as EPIC, have been developed which are of a size permitting their direct use in devices originally designed for the zero generation image intensifier tubes. However, these devices are substantially more expensively manufactured by the EPIC process. Thus, a need has arisen for a method of constructing the first and second generation tubes to permit their use in devices designed to accept the zero generation tubes without requiring a complete reworking of the optics geometry of the tubes or a reworking of the devices in which they are used.